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How to measure declination with a compass

A rough measure of the declination (compass orientation) can be simply made by … a compass. Verify that its needle is not perturbed by the proximity of ferrous metals and present the north/south axis of the graduation perpendicularly to the vertical plane of the dial or, if it is inclined, align the compass perpendicular to a horizontal line on the dial, as shown in the figure. For a horizontal dial, align the north/south axis of the compass with the y axis.

The type of declination (east or west) tallies with the side where the north point of the needle is located. Also:
o   If the point is on the east side, it indicates the value of the declination directly
o   If it is on the west side, subtract the value indicated by the point of the needle from 360° to get the declination

In the case shown in the diagram, the declination is 25° east.

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